Method and apparatus for continual compilation of a well data log

ABSTRACT

A method and apparatus for obtaining a data indication log while drilling a well, such data log providing lithology indication of subsurface strata while the actual drilling operation proceeds. The apparatus consists of plural sensing and computational equipment interconnected so that well fluid conductivity readings, well fluid circulation rate, well fluid temperature, and drill pipe penetration rate are continually monitored in relation to constant data as to borehole size, drill pipe size and the volume of the bottom assembly equipment; all effective variables and constants being continually calculated to provide an output log which is comparable to a resistivity log, calculated data being output at the well-site to provide continual substrata indications relative to the drilling operation.

United States Patent Haden Aug. 19, 1975 3,530] l() 9/ l 970 Beloglazov73/153 Primary ExaminerJerry W. Myracle Attorney, Agent, or FirmWilliamJ. Miller [75] Inventor: Elard L. Haden, Ponca City, Okla.

[73] Assignee: Continental Oil Company, Ponca ABSTRACT City, Okla. Amethod and apparatus for obtaining a data indica- [22] Fikid: July 31972 t on log while dr1ll1ng a well, such data log providmg lithologymdlcatlon of subsurface strata wh1le the ac- [21] Appl. No.: 268,399tual drilling operation proceeds. The apparatus consists of pluralsensing and computational equipment 52 us. c1. 73/153 f fluld f read [511 E2) 47/00 mgs, well fluid c1rculat10n rate, well fluld temperature, 58Field of Search 73 153; 175 and i penetrm'on rate are Commuuny 9 tored1n relation to constant data to borehole s1ze, [56] References Citeddrill plpe size and the ,volurne ofthe bottoin aseembly I r H equlpment,all cffectne Vdfldblfih and conhtants bemg LNlrtD STATES PATENTScontinually calculated to provide an output log which 3314974 9/1940Hayward 73/153 is comparable to a resistivity log, calculated data being3389b 7/1942 sufzm 4 73/153 X output at the well-site to providecontinual substrata 2,346,203 4/1944 ZillkOWSk) t. 73/153 indicationsrelative to th Operation- 3,386,286 6/1968 Moore 73/153 3,462.76]10/1969 Horeth et al H 73/153 X 8 Claims, 4 Drawing Figures 26 MUD paw/p2@ l f 22 MUD I -44 W4 M l 4 f 1/ J 5a 4a 4a 69 L 5 73 D/FFGREA/T/ALOIFFEEEA/ 77,44 MUD AMPL/F/EE AMPLIFIER WWW/w" COA/SMA/TS PM I 50reMpae/z was 54 a2 ao l 7 FLU/D FLU/0 DRILL PM? CON/WNW), C/ECgjfig/OA/PEA/Efflg/ON 69 L i i 40 HOLE 0,4771 COMPUTER M4 95 Dfi/LL -l08 p/ z-SIZE 02/22. 5 M L VOL UME RL'CUE/JIE METHOD ANII) APPARATUS FORCONTINUAL COMPILATION OF A WELL DATA LOG BACKGROUND OF THE INVENTION l.Field of the Invention The invention relates generally to the monitoringof well drilling operations and. more particularly, but not by way oflimitation. it relates to a method for continually compiling ameaningful data log during the drilling operations.

2. Description of the Prior Art The prior art includes numerous methodsand types of apparatus for compiling downhole log records of a wellbore. but most previous teachings in this technology have utilized downhole devices utilized as a separate operation with the borehole cleared.Thus. it was mandatory to pick up the drill pipe and drilling equipment.a lengthy and expensive operation, and then utilize a separate equipmentincluding cable and rigging to position various forms of boreholesensing device down within the hole in order to effect readings. Suchtesting procedures, or logs as they are referred to in the art. were ofvaried nature which included earth sample logs. mud logs. electric logs,radioactivity logs. and miscellaneous logs capable of giving acharacteristic read ing relative to various substrata encountered alongthe borehole. Included within the miscellaneous classification are suchas acoustic logs. caliper logs, temperature logs. dipmeter surveys.etc.. all of which required down hole equipment which. of necessity. hadto be lowered into the borehole as a separate and time-consumingoperation in order to effect sensing and transmit the usable readings toa surface meter or recording instrument.

There have been recent attempts to compile porosity logs and porepressure logs through normalization of drilling data at the rig sitewhile the drilling operation was proceeding. A gas saturation log hasalso proven feasible utilizing the similar approach. However, there hasnot yet been devised a suitable method or structure for keeping acontinual resistivity or conductivity log. herein termedpseudo-resistivity log. which maintains a running data compilationduring actual well drilling procedure. i.e. simultaneously with wellfluid or mud circulation, drill bit activation, etc.

SUMMARY OF THE INVENTION The present invention contemplates a method andapparatus for compiling a data log during drilling operations to providecontinual indication of borehole lithology. In a more limited aspect.the invention consists ofa plurality of sensing devices each adapted forderiving indication of specific variable parameters of the drillingoperation. including mud conductivity. temperature. mud flow rate. mudparticle content. etc.. such sensing outputs being processed and adaptedfor input to a computational device which also receives constant datapertaining to the bore hole and down hole equipment. A computationalequipment either analog or digital. then evaluates all data in properrelationship to maintain a continuous log of desired bore hole datawhile actual drilling takes place.

Therefore, it is an object of the present invention to provide methodand apparatus for deriving a selected data indication of boreholelithology during actual drilling procedure.

It is also an object to provide a method for deriving relative dataindication of sub-strata along a borehole at greatly reduced expense.

It is still another object to enable derivation of a lithology logindication of pseudo-resistivity. temperature and the like without thenecessity of a separate. time'consuming operation.

Finally, it is an object of the invention to provide apparatus forcontinually identifying subsurface strata onsite during the drillingoperation and eliminating the need for various other separate loggingoperations.

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic blockdiagram of a system constructed in accordance with the presentinvention;

FIG. 2 is a block diagram of one form of resistivity derivationcircuitry as may be utilized in the present invention;

FIG. 3 is a block diagram of an altemative form of resistivityderivation circuitry; and

FIG. 4 is a flow diagram of data procedure which may be utilized on sitefor compiling a pseudo-resistivity log.

DETAILED DESCRIPTION OF THE INVENTION As shown in FIG. 1. a data log maybe compiled at a drilling rig through utilization of a system 10situated at the drilling site adjacent a well bore 12 and the relatedsurface assembly I4. Surface assembly 14 is representative of the usualsurface drilling equipment as is well-known for well drillingoperations.

Included with the surface equipment are a drilling fluid or mud pump 16receiving mud via flow line 18 from a mud reconditioning equipment 20which may take the form of the various well-known apparatuses, i.e.shale shakers. cyclone separators, and the like. Drilling mud returningfrom borehole 12, as by upward travel through the casing annulus. flowsvia outcoming mud flow line 22 to a suitable reservoir or mud pit 24which may include provision for cuttings settling and the like. and thedrilling mud from mud pit 24 is passed through flow line 26 to the mudreconditioning equipment 20.

In conventional manner, drilling mud at predetermined pressure is drivenby the mud pump 16 through entering mud flow line 28, i.e. including thestand pipe. rotary hose. swivel. etc. (not shown), for passage throughthe rotary table and down through the drill pipe string 30. At thebottom of borehole 12, the drilling mud is passed internally through adrill collar 32 for entry into a selected form of drilling bit assembly34 whereupon released drilling mud with cuttings pick-up flows upwardthrough the borehole or casing annulus 36 for exit through the returnmud flow line 22 to the mud pit 24.

The data logging system 10 includes an on-site computer 40 of general orspecialized type. and a plurality of sensing and data input devices eachdesigned to derive a specific variable parameter for input to computer40 in whatever the specified data or electrical signal form. Anindication of resistivity is derived through a pair of conductivitysensors 42 and 44 which provide outputs via lines 46 and 48 todifferential amplifier 50.

The conductivity sensors 42 and 44, specific structure to be furtherdescribed below, derive conductivity readings from the entering mud (tothe well) and the out-coming mud. respectively. to provide outputindication via lines 46 and 48 to differential amplifier circuitry 50.The in-going mud conductivity reading is ap plied through a conventionalstorage stage 51 to be delayed by the time of mud circulation throughthe downhole portion of the flow. Thus. comparison readings are made forthe same portion of mud at the two selected points of its circuit.Storage 51 is controlled from computer as will be further described.Differential amplifier then provides an output via line 52 which mayindicate conductivity difference, and a fluid conductivity stage 54provides suitable output on line 56 to computer 40 which is anindication of relative conductivity G (or the reciprocal resistivity R)of the mud as it includes drilled cuttings. particles, sediment, etc.Such relative conductivity indication is directly relatable toresistivity.

In order to give reliable credence to such relative conductivity orresistivity indication, it is necessary to take into consideration anumber of variable parameters as well as a plurality of constants sothat the resistivity indications can be made reliable and meaningfulrelative to the particular substrata which is being drilled at thattime. Thus, data derived from the mud flow line is assembled to indicatethe rate of drilling fluid or mud flow through analysis of data as tothe rate of drill pipe penetration and various constants such as borehole diameter, drill pipe size and the volume of the bit assembly anddrill collar.

A temperature indication is derived from a suitable sensor 58, e.g. athermocouple in contact with the mud, as inserted in the entering mudline 28 to provide indication via line 60 to a differential amplifiercircuitry 62. A second temperature sensor 64 in out-coming mud line 22provides indication of returning mud temperature via line 66 foralternate input to differential amplifier 62. lngoing mud temperatureindication is stored for circulation time in storage 67 under control ofcomputer 40. Practically. storage stages 51 and 67 may be comprised of aunitary device of conventional design under computer control asindicated by line 69. An amplified difference signal is then present atlines 68 for input to a temperature stage 70 which prepares or formatsthe differential output indication for input via line 72 to computer 40.Temperature stage 70 may be any of various well-known data preparationstages which transform the differential output into proper constitutionfor input in accordance with requirements of computer 40. Thus, it maybe an analog operational amplifier stage or an analog to digitalconverter stage, depending upon the type of computational equipmentutilized at computer 40.

The drilling fluid or mud circulation rate is also input to computer 40as derived directly from mud pump 16 by a suitable sensing device 74.Sensing device 74 may be a conventional form ofdirect metering outputwhich provides electrical output on line 76 by reading directly from therotational components of mud pump 16. Mud flow indication on lead 76 maythen be applied to such as an integrator 78 to provide an average flowindication whereupon such output is applied via line 80 to a fluidcirculation rate stage 82 to provide output rate information in properformat or signal form for input via line 84 to computer 40. Stillanother variable, drill pipe penetration rate may be sensed by asuitable sensing device 86 to provide indication via line 88 through adrill pipe penetration rate formatting stage 90, and the properlyprepared rate data or signal is supplied via line 92 to computer 40. Thepenetration rate sensing device 86 may be such as a conventional form ofpulsing device providing pulse output in proportion to downward drillpipe movement or, alternatively, the drill pipe penetration data can beperiodically updated through a simple manually operated device withinaccess of the rig floor personnel.

It has also been found that in some cases it is necessary to enter stilladditional variable parameters relating to the mud dilution factor orparticular drilling fluid constituencies. A dilution factor may becalculated using input constant data to determine earth material drilledper unit time or per foot. In many areas the percentage of cuttingsrecovered will decrease with depth. Thus, in order to account for thedilution of drilled material as penetration rates change, an evaluationof the amount of cuttings recovered at the shale shaker or such in mudconditioning 20 can be conveyed via suitable mud constants flow stage 94for input to computer 40. Derivation of the mud constants may beautomatic, by suitable sensing and signal determinative stages, or theinformation can be derived through continuous or periodic weighing ofcuttings recovered on the shale shaker followed by appropriate datainput to computer 40.

Constant data is also input to computer 40 via hole data stage 96, drillpipe size stage 98 and bottom as sembly volume stage 100. The hole datastage 96 merely inputs the constants relating to the borehole diameterversus the length of the borehole, the volumetric consideration of whichprovides mud volume when considered in relation to drill pipe insidediameter and the volume displaced by drill collar 32 and bit assembly34.

Computer 40, being continually supplied with all variable and constantparameters, is able to provide an output indication of conductivity, orthe reciprocal resistivity, continually and in relation to the bitcontact area 102 as previously penetrated. That is, arrival of fluidfrom the hole bottom may take as much as an hour or more. Simultaneousmud resistivity readings or temperature indications must be delayed by atime which is a function of mud flow volume down the borehole and thefluid or mud circulation rate as derived from mud pump 16. Such delayedcomparison is effected either by the inherent capabilities of computer40, or by active storage stages 41 and 67, as will be further describedbelow. Output from computer 40 may then be present on line 104 to aconventional form of log recorder 106 which will provide a continuousindication of pseudo-resistivity, temperature or such versus boreholedepth. it may also be desirable to record indication of all or selectedones of the variable parameters relative thereto. and further forms ofoutput from computer 40, may be made available via line 108 to a dataoutput device 110. Data output device 110 may be such as a wellknownform of computer printout mechanism or a recorder providing an output inselected dif ferenc coordinates.

The data logging system 10 may be utilized to provide relative dataindications of various parameters. Thus, resistivity data may becompiled to generate what may be termed a pseudo-resistivity log of theborehole.

Temperature corrections may be utilized in compilation of thepseudo-resistivity log. or a meaningful log may be constructed primarilyutilizing temperature data alone. Recent findings indicate that atemperature log can be one of the best indicators of geo-pressure alonga borehole.

FIG. 2 illustrates a particular form of circuitry wherein resistivitymay be derived continually for a determinable bottom hole depth.Resistivity data is derived for out-coming mud flow line 22 forcomparison with prior derived resistivity data from entering mud flowline 28. Proper delay insures that differences of resistivity data willbe continually derived for essentially the same unit portion of thefluent mud material. The out-coming mud flow will of course containcuttings, sediment and other solid and fluid materials which will serveto alter the resistivity data, such alteration being indicative of theform of strata at the borehole bottom, i.e. the penetration point at thetime the particular material was drilled loose.

A suitable form of d-c generator 1 provides output current at adesignated d-c potential on lead 112 for connection to each ofelectrodes 114 and 116, in the entering and out-coming mud flow lines,respectively. Lead 112 is also connected through each of equal valuecurrent limiting resistors 118 and 120 to respective reference inputs ofd-c amplifiers 122 and 124. An additional pair of sensing electrodes 126and 128, disposed in the mud flow lines opposite the respectiveelectrodes 114 and 128, provide inputs 130 and 132 to respective senseinputs of d-c amplifiers 122 and 124. Thus, the d-c amplifiers 122 and124 operating from the same reference provide output in the form ofa d-csignal output wherein the amplitude of the output signal is a directindication of the difference in conductivity between the two flow lines.

Output from (Le amplifier 122 is then applied through a switch 126:! toa suitable storage 134 which is operated under control of a circulationrate generator 136 to provide signal storage for the proper delay timeof mud circulation. Switch 126ub is provided in order to illustrate thatcertain forms of operation will allow disabling of entering mud flowsensing, as there are many operations which will permit sensing ofoutcoming mud flow only thereby to derive meaningful resistivityinformation.

The storage 134 may be any of various analog devices, e.g. an endlesswire or tape recorder synchronized as to desired mud circulation timethrough either transport speed control or read out head position ascontrolled in response to circulation rate generator 136. Thecirculation rate generator. driven as to pulse rate by the mud pump, andserving to control the speed of the recorder of storage 134 as afunction of pulse repetition rate, such circuitry and storage techniquebeing well-known in the art. Delayed output from storage 134 is thenpresent on lead 138 to one input of a differential amplifier 140. Anoutput d-c signal from outcoming mud amplifier 124 is present on a lead142 to the remaining input of differential amplifier 140. A referenced-c voltage is applied from current limiting resistor 118 via lead 144to provide a reference bias to differential amplifier 140. The switchsection 12617 (shown open) can be actuated to interconnect leads 144 and138 and provide reference potential to a reference input of differentialamplifier 140 when the circuitry is operated in a single flow line modeof operation, as previously referred to.

Output from the differential amplifier is in the form of a differencesignal proportionate to the difference in conductivity as betweenentering mud containing no cuttings and the same unit portion of mudmaterial when detected as out-coming mud flow. The differentialconductivity signal on a lead 146 may then be applied to a suitable formof operational amplifier 148 which serves to provide a reciprocaloutput. or the equivalent of a relative resistivity measure, as presenton output lead 150. Such resistivity measure can also be furtherevaluated relative to the mud dilution factor for correction to astandard condition, e.g. cubic feet of rock per barrel of mud.

An alternative output on lead 146 may be applied to an analog/digitalconverter 152 such that coded, digital output is present on a lead 154to terminal 156. A digital indication of conductivity difference atterminal 156 may then be utilized variously for further data derivation.Thus, digital output at terminal 156 may be applied to suitableformatting circuitry of well-known type to prepare digital data forinput to the associated computer equipment. In the case of utilizationof an analog computational machine, the time-analog signal present atthe lead 150, and indicative of relative resistivity, can be applieddirectly for input to the computer as a time-varying continuousparameter.

FIG. 3 illustrates an alternative form of circuitry for processing theresistivity or conductivity data, such digital circuitry probably beingmore readily adaptable into the frame work of equipment presently usedin the art. A change in voltage, A15 indicative of mud flow entering thewell, for example derived from d-c amplifier 122 of FIG. 2, may beapplied at input 160 to analog to digital converter 162 which convertsthe AE signal to digital form of preselected bit representation forinput to digital storage 164. Digital storage 164 may be any of theconventional digital storage devices, tape, core, counter array, etc. AAE signal indicative of mud flow out-coming from the well, is applied atan input 166 to analog to digital converter 168, and the output fromconverter 168 is applied directly to a difference network 170.

Each of analog to digital converters 162 and 186, and digital storage164 are maintained under the control of clock circuit 172 via lead 174,and a circulation rate generator 176 is also controlled in accordancewith the output of clock 172. Clock 172 may be any conventional form ofpulse generator having the desired repetition rate for control ofconverters 162 and 168 and digital storage 164. The circulation rategenerator 176 may be constituted of conventional pulse delay circuitrywherein pulse output is delayed for a predetermined time, in this casethe time as derived for mud circulation from the entering sensing pointto the outcoming sensing point, whereupon a pulse output is provided vialead 178 for input to digital storage 164 to output the delayed AE (mudflow entering) data via line 180 to the difference network 170.

Each of analog to digital converters 162 and 168, and digital storage164 are maintained under the control of clock circuit 172 via lead 174,and a circulation rate generator 176 is also controlled in accordancewith the output of clock 172. Clock 172 may be any conventional form ofpulse generator having the desired repetition rate for control ofconverters 162 and 168 and digital storage 164. The circulation rategenerator 176 may be constituted of conventional pulse delay circuitrywherein pulse output is delayed for a predetermined time, in this casethe time as derived for mud circulation from the entering sensing pointto the outcoming sensing point, whereupon a pulse output is provided vialead 178 for input to digital storage 164 to output the delayed AE (mudflow entering) data via line 180 to the difference network 170.

Difference network 170 serves to receive the instant out-coming mud AEdigital signal simultaneous with the properly delayed entering mud AE,digital signal to derive a difference digital signal as output on a line182 to the related output devices, in this case an R data output 184 anda selected form of recorder 186. Thus, the AE- data could be directlyrecorded at this stage as a relative indication of resistivity, or thedata could be applied to R data output 184 for derivation ofconductivity G or the reciprocal resistivity R, for further applicationas input to the associated computer equipment located on-site.

Most accurate compilation and recording of data is probably enabledthrough utilization of a digital processing scheme. Also, digitalcomputer equipment of either general or specialized purpose type isreadily available in the art at present. Therefore, a total digitalscheme utilizing an on-site digital computer and associated peripheralreadout recorder may be the primary selection. FIG. 4 illustrates a dataflow diagram which will enable utilization of such digital computationalequipment.

Input conductivity data is presented at input stage 200, and this couldbe such data as derived at R data output stage 184 of FIG. 3. Thecomputational scheme may be effected utilizing either conductivity orresistivity data, or temperature data as will become apparent. Theconductivity quantities are reciprocal, and such data. as properlyformatted and applied to input stage 200, would then be processedthrough a processing stage 202 which would include inputting onconstants relative to hole size. drill pipe size, bottom assembly volumeand the like. This annotation stage merely sets the constant parametersrelative to the borehole and connecting mud line length, if any exists,between the upper borehole casing and the point of sensing; and theseconstants determine the total amount of drilling fluid or mud volumewithin the operating system which, in turn. will enable calculation ofthe circulation time of a given unit portion of mud. In the case wheretemperature log data is to be established. the data is input throughflow stage 202 via line 203, as will be further discussed.

The process then proceeds to processing stage 204 wherein finaldetermination as to mud volume is made through consideration of theinstantaneous vertical length of the drill pipe. i.e. the data for depthof penetration. Time data is input at stage 206 to a predefined processstage 208 wherein calculation is effected utilizing all input constantsand drill pipe vertical length to derive the drill pipe penetration raterelative to mud volume within the borehole. Flovv stage 210 their reccives input data relative to fluid circulation parameters. andpredefined process stage 212 calculates total drilling fluid flow ratethrough the predetermined portion of the flow system length.

Operational inputs as to temperature corrections and fluid data may alsobe considered at this point. Temperature data applied at input stage 214is passed through a stage 216 to determine any temperature correctionwhich may be required or advisable in the particular operation. Thus, ifthe data output is a parameter other than temperature, e.g. resistivity,affirmative output via line 217 from decision stage 216 suppliedtemperature correction data through the main process flow. If the finaldata, is a temperature log, data flow is via line 203 for input oftemperature data at the initial processing flow stage 202.

Similarly, fluid data derivation at input stage 218 and rock and cuttingdata from stage 220 may require alterations, and in such case theprocessing stage 221 determines fluid constants, or variations fromfluid constants, for further input to the main data flow. Input to stage222 considers such variables as drilled cuttings removal, amount of rockdrilled, amount of material dispersed, etc. The total fluid flow rate ascalculated in predefined processed stage 212, as well as any temperaturedata correction and/or fluid data correction, are input to a predefinedprocess stage 222 wherein final calculation is made as to the finalbottom hole fluid data, i.e. a fluid resistivity, temperature or suchselected measurement indicative of a relative data value at thepredetermined strata.

Data output from flow stage 222 is then output in selected form, as forexample to a suitable form of storage 224 and/or print out device 226,to provide a logtype continuous record of the printing operationrelative to relative resistivity or pseudo-resistivity indication asderived throughout the drilling process. Other ancillary data asindicated by stage 228, such as substrata soil type, time notations,porosity indications, gas content indications, and the like may beentered for printout on the final record within stage 226. It shouldalso be understood that the well parameter evaluations as variouslyderived heretofore, and as particularly directed to resistivity andtemperature data, can also be used to identify and correct othermeasurements, e.g. redox potential, pH, specific ion concentration,color,

etc.

The foregoing discloses novel method and apparatus which enablescompilation of a relative data indication or well bore log continuously,at the well site, throughout a drilling operation. The present inventionenables compilation of valuable information as to lithology during thedrilling operation and is capable of calculating various factorsincluding an accounting for time difference of measurements taken (i.e.as between entering and outcoming drilling fluid), and the system canmake corrections for temperature, pressure changes, changing penetrationrate versus the mud dilution factor, etc. Any and all calculatable datainherently present relative to the operation ma be input to the finalcompilations; however, the number ofthese factors may be specificallylimited in order to obtain a designated form of highl reliable relativeresistivity or pseudo-resistivity (or temperature) log of the boreholeChanges may be made in the combination and arrangement of elements asheretofore set forth in the specification and shown in the drawing; itbeing understood that changes ma v be made in the embodiments disclosedwithout departing from the spirit and scope of the invention as definedin the following claims.

What is claimed is:

l. A method for deriving a continuous pseudoresistivitv log indicationof selected sub-strata through which a bore hole is being formedutilizing drilling equipment with drilling fluid circulation equipment.comprising the steps of:

continually deriving first electrical resistivity measurements from thedrilling fluid entering said bore hole;

storing said first electrical resistivity measurements for a durationequal to circulation time ofa drilling fluid.

continually deriving second electrical resistivity measurements from thedrilling fluid out-coming from said bore hole;

deriving a difference resistivity between said stored first electricalresistivity and the second electrical resistivity measurements:

continually deriving an indication of drilling penetration depth. andperiodically deriving an indication of penetration depth change;

generating an indication of the total drilling fluid volume in the borehole as well as the drilling fluid circulation rate a function of timeto determine the amount of time for a unit portion of drilling fluid tocirculate from the bore hole bottom to the bore hole top; and

recording said differential resistivity measurement on a time basedelayed by said amount of time for a given portion of fluid to circulatefrom bore hole bottom to bore hole top in order to provide an out putdisplay of electrical resistivity versus bore hole depth.

2. A method as set forth in claim 1 which includes the steps of:

deriving a drilling fluid dilution factor which is a coefficientindicating the relative consistency of said drilled borehole material;and

applying said dilution factor to said difference resistivity forrecording such that a pseudo-resistivity log is compiled for saidborehole.

3. A method for deriving a pseudo-resistivity log of sub-strata throughwhich a bore hole is being formed utilizing drilling and drilling fluidcirculation equipment. such pseudo-resistivity log being compiled usingan automatic electronic data processing machine. the method comprisingthe steps of:

sensing entering drilling fluid to said bore hole at a first sensingpoint to derive first relative resistivity data for drilling fluidpresent at said second sending point; storing said first relativeresistivity data for a duration equal to circulation time of saiddrilling fluid;

sensing out-coming drilling fluid from said bore hole at a secondsensing point to derive second relative resistivity data for drillingfluid present at said second sensing point:

comparing said stored first resistivity data and said second resistivitydata to derive a difference resis tivity data;

sensing bore hole penetration depth for evaluation with constant data asto bore hole diameter and volume of bore hole drilling equipment toderive total drilling fluid volume in circulation for input to said dataprocessing machine;

sensing the rate ofdrilling fluid circulation for input to said dataprocessing machine;

deriving time delay data representative of the time during which a givenunit portion of drilling fluid will progress from the bore hole bottomto the outcoming drilling fluid sensing point; and applying saiddifference resistivity data to said data processing machine to generatean output indication constituting a pseudo-resistivity log ofresistivity which is directly related to bore hole penetration depth. 4.A method as set forth in claim 3 which is further characterized toinclude:

sensing the difference in temperature of entering and out-comingdrilling fluid and entering the tempera ture change data into said dataprocessing machine as a periodic correction factor to output saidpseudo-resistivity log as a function of temperature. 5. A method as setforth in claim 3 which is further characterized to include:

deriving a dilution factor proportional to size and amount of drilledearth material present in said out-coming drilling fluid and enteringsaid dilution factor into said data processing machine as a periodiccorrection factor to output said pseudoresistivity log as a function ofsaid dilution factor. 6. Apparatus for deriving a pseudo-resistivity logof earth sub-strata through which a bore hole is being formed byutilization of drilling and drilling fluid circulation equipment.comprising:

first sensing means including an electrical power source for deriving afirst resistivity indication from the drilling fluid entering said borehole; means for generating output indication of the circulation time ofa unit portion of drilling fluid in said fluid circulation equipment;storage means for storing said first resistivity indication for aduration equal to the circulation time of said drilling fluids; secondsensing means including an electrical power source for deriving a secondresistivity indication from the drilling fluid out-coming from said borehole; comparator means deriving a difference resistivity indication fromsaid stored first resistivity indica tion and the second resistivityindication; means for indicating the instantaneous depth of said borehole: means including a recorder receiving said difference resistivityindication and providing an output record of resistivity in relation toinstantaneous bore hole depth. 7. Apparatus as set forth in claim 6which is further characterized to include:

means sensing temperature of entering drilling fluid; means storing saidtemperature indication for a duration equal to said circulation time;means sensing temperature of out-coming drilling fluid; and means forcomparing and deriving differential tem perature and recording outputindication of temperature versus bore hole depth. 8. Apparatus as setforth in claim 7 which is further characterized in that:

said means including a recorder for providing output record of saidresistivity and temperature is a digital data processing machine withperipheral display.

1. A method for deriving a continuous pseudo-resistivity log indicationof selected sub-strata through which a bore hole is being formedutilizing drilling equipment with drilling fluid circulation equipment,comprising the steps of: continually deriving first electricalresistivity measurements from the drilling fluid entering said borehole; storing said first electrical resistivity measurements for aduration equal to circulation time of a drilling fluid; continuallyderiving second electrical resistivity measurements from the drillingfluid out-coming from said bore hole; deriving a difference resistivitybetween said stored first electrical resistivity and the secondelectrical resistivity measurements; continually deriving an indicationof drilling penetration depth, and periodically deriving an indicationof penetration depth change; generating an indication of the totaldrilling fluid volume in the bore hole as well as the drilling fluidcirculation rate as a function of time to determine the amount of timefor a unit portion of drilling fluid to circulate from the bore holebottom to the bore hole top; and recording said differential resistivitymeasurement on a time base delayed by said amount of time for a givenportion of fluid to circulate from bore hole bottom to bore hole top inorder to provide an output display of electrical resistivity versus borehole depth.
 2. A method as set forth in claim 1 which includes the stepsof: deriving a drilling fluid dilution factor which is a coefficientindicating the relative consistency of said drilled borehole material;and applying said dilution factor to said difference resistivity forrecording such that a pseudo-resistivity log is compiled for saidborehole.
 3. A method for deriving a pseudo-resistivity log ofsub-strata through which a bore hole is being formed utilizing drillingand drilling fluid circulation equipment, such pseudo-resistivity logbeing compiled using an automatic electronic data processing machine,the method comprising the steps of: sensing entering drilling fluid tosaid bore hole at a first sensing point to derive first relativeresistivity data for drilling fluid present at said second sendingpoint; storing said first relative resistivity data for a duration equalto circulation time of said drilling fluid; sensing out-coming drillingfluid from said bore hole at a second sensing point to derive secondrelative resistivity data for drilling fluid present at said secondsensing point; comparing said stored first resistivity data and saidsecond resistivity data to derive a difference resistivity data; sensingbore hole penetration depth for evaluation with constant data as to borehole diameter and volume of bore hole drilling equipment to derive totaldrilling fluid volume in circulation for input to said data processingmachine; sensing the rate of drilling fluid circulation for input tosaid data processing machine; deriving time delay data representative ofthe time during which a given unit portion of drilling fluid willprogress from the bore hole bottom to the out-coming drilling Fluidsensing point; and applying said difference resistivity data to saiddata processing machine to generate an output indication constituting apseudo-resistivity log of resistivity which is directly related to borehole penetration depth.
 4. A method as set forth in claim 3 which isfurther characterized to include: sensing the difference in temperatureof entering and out-coming drilling fluid and entering the temperaturechange data into said data processing machine as a periodic correctionfactor to output said pseudo-resistivity log as a function oftemperature.
 5. A method as set forth in claim 3 which is furthercharacterized to include: deriving a dilution factor proportional tosize and amount of drilled earth material present in said out-comingdrilling fluid and entering said dilution factor into said dataprocessing machine as a periodic correction factor to output saidpseudo-resistivity log as a function of said dilution factor. 6.Apparatus for deriving a pseudo-resistivity log of earth sub-stratathrough which a bore hole is being formed by utilization of drilling anddrilling fluid circulation equipment, comprising: first sensing meansincluding an electrical power source for deriving a first resistivityindication from the drilling fluid entering said bore hole; means forgenerating output indication of the circulation time of a unit portionof drilling fluid in said fluid circulation equipment; storage means forstoring said first resistivity indication for a duration equal to thecirculation time of said drilling fluids; second sensing means includingan electrical power source for deriving a second resistivity indicationfrom the drilling fluid out-coming from said bore hole; comparator meansderiving a difference resistivity indication from said stored firstresistivity indication and the second resistivity indication; means forindicating the instantaneous depth of said bore hole; means including arecorder receiving said difference resistivity indication and providingan output record of resistivity in relation to instantaneous bore holedepth.
 7. Apparatus as set forth in claim 6 which is furthercharacterized to include: means sensing temperature of entering drillingfluid; means storing said temperature indication for a duration equal tosaid circulation time; means sensing temperature of out-coming drillingfluid; and means for comparing and deriving differential temperature andrecording output indication of temperature versus bore hole depth. 8.Apparatus as set forth in claim 7 which is further characterized inthat: said means including a recorder for providing output record ofsaid resistivity and temperature is a digital data processing machinewith peripheral display.